The present disclosure relates to electronic systems and methods, and in particular, to circuits and methods for driving resonant actuators.
FIG. 1 illustrates an example of electro-mechanical actuator 100. Electro-mechanical actuators typically include an inductive coil, such as a voice coil, a magnet, a mass, and a spring. This example shows a linear resonant actuator, where a magnetic field is generated by driving coil 101 with a drive signal on wires 102a and 102b. The magnetic field interacts with a magnet 105 in a central region 104 of a mass 103. The magnetic field creates a force to move mass 103. The mass 103 and magnet 105 are suspended on a spring 106 inside a casing 108 and 109. As the magnetic field varies with the applied drive signal, the magnet and mass move as they interact with the spring, which creates a vibration.
Resonant actuators may be modeled as a high-Q mechanical vibration module that has a particular resonant frequency. Traditionally, drive circuits have attempted to drive actuators at the resonant frequency to achieve a desired mechanical vibration. However, a variety of factors may cause the resonant frequency to drift. Drift may occur due to temperature, aging, orientation, and mechanical tolerances, for example. In some cases, drift may change the resonant frequency by +/−10% for the combined effects mentioned above. When the drive frequency differs from the resonant frequency by even 2-3%, the vibration strength may drop by as much as 50%. Therefore, maintaining desired vibration strength may require more power when the drive frequency is misaligned with the resonant frequency. The excessive power not only reduces the system efficiency but may also pose reliability issues due to overheating of the coil.